Adding an extra domain is simple. Just log in to your control panel, go to the domain section and click the "Add domain" link.
Next, enter the name of your domain and you're done.

What if I want my domains to go to totally separate web sites?

No problem. The control panel allows you to easily assign each domain to the directory of your choice.

Is there any extra cost for this?

Of course not! Adding extra domains to your account is free. Keep in mind that you must of course already have registered the domain name.

Are there any limits to the number of domains I can host?

We do not place any preset limit on the number of extra domains. Of course like everything else, the goal is fairness and server stability. If one of your domain's consumes a large amount of resources, you should consider put it on a separate account a VPS.